Naming the Forces
What is Friction?
Friction is the resistance to motion
created by two objects moving against
each other.
Also, friction creates heat.
Try to rub your hands together! Did you
notice that your hands are now warmer?
This is all due to friction!
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Displaying friction on force diagrams
Remember: Friction acts in the
opposite direction of motion.
Driving Force
Friction
Try the next one yourself!
This bus is moving backwards (reversing). Mark with a red
arrow the friction acting on this bus.
Driving Force
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What are Weight & reaction?
We have already seen how one of the
non-contact forces that exists is the
gravitational pull of the earth.
The weight of an object is the force
by which the earth is pulling it down.
It always points downwards.
The reaction is the opposing force
applied by surfaces on objects.
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What is the difference
between weight & Mass?
The mass of an object describes how
heavy an object is. The SI units for
mass are kilograms (kg).
However the weight of an object
describes how hard gravity is pulling
on it.
Hence, Weight is considered to be a
force and therefore measured in
newtons (N).
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What is air resistance?
Air Resistance is another force
which opposes motion.
The larger the area of the object,
the greater the resistance.
Example: Look at these parachutes below.
Parachute 1
Parachute 2
The parachute on the left has a smaller area than the
parachute on the right.
Therefore as can be seen from the size of the arrows,
there is more air resistance exerted on the second
parachute than on the first one.
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What is Upthrust?
Upthrust is the force that enables
things to float in water or hover in the
air.
This force acts against the weight of
the object as seen below.
If the weight is equal to the upthrust
then the object floats.
If the weight is greater than the
upthrust it will sink.
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What do you think happens if the upthrust is greater than
the weight?
Which method of transportation uses this concept?
Summary:
Forces can be divided into two categories:
1) Contact Forces 2) Non-Contact Forces.
Forces can make objects:
 Start to move
 Stop moving
 Accelerate
 Decelerate (slow down)
 Change the direction of motion
 Deform
Forces can be displayed using a diagram.
 In these diagrams we draw an arrow to represent
the force.
 The direction of the arrow shows the direction of
the force.
 The width of the arrow represents the size of the
force.
Some of the most important forces that we experience
in our lives are: Friction, weight, reaction, air resistance
and uphtrust.
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